The LCMV virus usually causes little or no illness in healthy people, but can be deadly for those with weak immune systems like cancer patients and transplant recipients, who take immune-suppressing drugs to prevent organ rejection. Organs are routinely tested for many viruses, but there is no commercial test for LCMV.

Though there's no evidence that the deaths are anything but rare, recent discoveries that rabies and West Nile virus can spread through donated organs has officials worried that the latest virus might have gone undetected before now.

"We don't know how commonly it occurs," said Dr. Matthew Kuehnert, assistant director of blood safety for the federal Centers for Disease Control and Prevention. "We're learning as we go here. This is a new phenomenon."

Rhode Island and Massachusetts officials said Monday they are investigating the deaths of three people who got infected organs from a female donor whose pet hamster tested positive for lymphocytic choriomeningitis virus, or LCMV. A fourth organ recipient is believed to be recovering.

On Tuesday, health officials in Wisconsin revealed that four transplant recipients died in the only previously known cases involving the virus in December 2003.

The cases weren't clear-cut -- the organ donor and a woman who received a lung from him in an operation in Minnesota both tested negative for LCMV. But three other transplant patients tested positive for the virus, strongly suggesting the donor was the source.

That was the only thing in common the recipients had, said James Kazmierczak, an epidemiologist for the Wisconsin Department of Health and Family Services.

About 5 percent of mice, hamsters and other rodents carry LCMV and about 2 percent of the general public has antibodies to it, meaning they've been exposed to it at some point, Kazmierczak said.

The virus usually causes little or no illness in healthy people, but can be deadly for those with weak immune systems like cancer patients and transplant recipients, who take immune-suppressing drugs to prevent organ rejection.

Organs are routinely tested for many viruses but there is no commercial test for LCMV. The CDC, state health officials, the Food and Drug Administration and others are investigating. Some sought to reassure people who need transplants.

"We would encourage people who are on the waiting list not to be concerned with this," said Rhode Island health director David Gifford. "This is an extremely rare and unusual event."

Wisconsin officials said they made no public statements in 2003 because of the tenuous evidence and because the virus doesn't spread person-to-person and wasn't considered a public health threat.

A doctor who treated one of the New England organ recipients said she didn't find out about the Wisconsin cases until too late to save her patient's life.

Dr. Staci Fischer, infectious disease physician for the transplant group at Rhode Island Hospital, discovered the infection in April, when two of her kidney transplant patients developed flu-like symptoms. One survived, but the other did not.

Fischer said she searched medical literature and asked colleagues around the country if they had heard of anything similar. But it wasn't until she called the CDC that the connection to LCMV and the donor's hamster was made.

"It clearly rang a bell in their minds," she said. "My heart sank. In the long run, it would have made a difference for our other patient."

Two patients in Massachusetts -- one a double lung recipient, the other a liver recipient -- also died within weeks of the transplants, which were performed on April 10 and April 11, according to the Rhode Island Department of Health.

Two people who received corneas from the same Rhode Island woman in operations outside the United States are being monitored for signs of illness, said Kuehnert, who declined to say where the corneas were sent.

The donor's hamster, bought at a Petsmart store in Warwick, tested positive for LCMV. More than 100 hamsters, mice and guinea pigs from the store were euthanized and shipped to the CDC in Atlanta, where all will be tested.